The present invention relates to an induction heating device having a hollow cylindrical body made of electrically conductive material, at least one primary induction coil and at least one auxiliary induction coil, the at least one primary induction coil and the at least one auxiliary induction coil being arranged in the cavity of the cylindrical body in such a way that, by means of the at least one primary induction coil, a first magnetic field can be produced in the cylindrical body and, by means of the at least one auxiliary induction coil, a second magnetic field can be produced in the cylindrical body. It relates moreover to a method of operating an induction heating device, which comprises a hollow cylindrical body made of electrically conductive material, at least one primary induction coil and at least one auxiliary induction coil, the primary induction coil and the at least one auxiliary induction coil being arranged in the cavity of the cylindrical body, in which method, firstly, by means of the at least one primary induction coil, a first magnetic field is produced in the cylindrical body and, by means of the at least one auxiliary induction coil, a second magnetic field is produced in the cylindrical body.
DE 195 32 044 discloses a device and method, in which, in addition to a primary induction coil for heating the cylindrical body, induction auxiliary coils are used which are intended to serve to prevent the production of a gap between the cylindrical body and connecting sections or to maintain the circular shape of the cylindrical body. To this end, before the actual commissioning, the auxiliary induction coils in the edge regions of the cylindrical body are used to reinforce the magnetic field produced by the primary induction coil. The time which is required until the surface temperature distribution on the cylindrical body is stabilized is shortened thereby. After the cylindrical body has been heated uniformly to the desired temperature, the supply to the auxiliary induction coils is switched off. According to the version represented in DE 195 32 044, after the uniform heating, the supply by the primary induction coil is on its own sufficient to ensure thermal equilibrium.
U.S. Pat. No. 5,990,461 discloses a thermal processor, in which a photothermographic film can be developed by means of a heated cylindrical body, a heating lamp being used as a heat source. A use of the induction heater described in DE 195 32 044 as a substitute for the heating lamp does not lead to any satisfactory result; the temperature distribution which results along the longitudinal axis of the cylindrical body during operation would lead to temperature differences on the area of the surface of the cylindrical body, on which the photothermographic film rests for development, said differences lying outside a tolerance band which would be permitted for the development of materials of this type. By means of comparative measurements, it was established that the temperature drop toward the ends of the cylindrical body during operation, that is to say after the auxiliary induction coils had been switched off, was about 20%, starting from the temperature in the central region of the cylindrical body. This would not be a problem if the cylindrical body were to be configured to be wide enough and the recording material to be developed were to be transported only in the central region of the cylindrical body, in which the temperature is sufficiently constant. With regard to the most compact device possible, however, it is desirable to make the length of the cylindrical body as short as possible.
It is therefore the object of the present invention to make possible a temperature distribution on the surface of a cylindrical body which, with a compact design, meets the requirements placed on the thermal development of a recording medium.
In the sense of the present invention, recording medium is to be understood in particular to mean material which can be developed by the thermal route, for example photothermographic material or thermographic material. In particular, it can be a film for radiographic applications.
The invention is based on the finding that the temperature distribution along the cylindrical body may be controlled very precisely if the two magnetic fields, which are produced firstly by the at least one primary induction coil and secondly by the at least one auxiliary induction coil, weaken each other. As a result, in the areas of the surface of the cylindrical body in which excessively high temperatures are produced in the cylindrical body because of the primary induction coil, the energizing magnetic field can be specifically reduced, in order as a result to arrive at a uniform temperature profile. By means of this measure, temperature profiles along the longitudinal axis of the cylindrical body on the outer surface of the cylindrical body can be achieved at which a deviation of only 2% or less results over 80% or more of the length of the cylindrical body. It is therefore possible for processors with induction heating devices to be implemented which can be used for the development of recording material even for highly sensitive applications, for example radiographic applications in medicine. In particular in this area, it is completely unacceptable if there is a risk of misdiagnosis as a result of non-uniform development.
As opposed to the induction heating device of DE 195 32 044, in the case of the induction heating device according to the invention, the operation of the auxiliary induction coils is possible independently of the preheating of the induction heating device during actual operation.
In a first embodiment, the cylindrical body has a longitudinal axis, and at least one primary induction coil is arranged parallel to this longitudinal axis. By using the at least one primary induction coil, a magnetic field that is homogeneous over its longitudinal extent can be produced. At least one auxiliary induction coil is arranged in the area of a central section of the primary induction coil. In this embodiment, the primary induction coil is dimensioned such that it generates the temperature necessary for the development in the edge regions of the cylindrical body. This would lead to an excessively high temperature in the central region of the cylindrical body, but this is reduced by the auxiliary induction coil arranged there.
In another embodiment, the cylindrical body likewise has a longitudinal axis. The at least one primary induction coil is again arranged parallel to this longitudinal axis, but it is now possible, by using the at least one primary induction coil, to produce a magnetic field that is inhomogeneous over its longitudinal extent, being stronger in a first and in a second edge region of the primary induction coil than in a central region. In each case, at least one auxiliary induction coil is arranged in the region of the first and of the second edge region of the primary induction coil. In this embodiment, the temperature in the edge regions of the cylindrical body is kept at the necessary level by the primary induction coil producing a stronger magnetic field there which then, if the temperature becomes too high, can be weakened to the necessary values by the auxiliary induction coils arranged in the edge region.
It is preferable for the at least one auxiliary induction coil to be capable of activation by being short-circuited. This implementation provides the advantage that it is possible to dispense with an additional power supply unit for driving the at least one auxiliary induction coil.
Preferably, a control device can further be provided, in order to activate at least one auxiliary induction coil. The control device can comprise a look-up table, in which the characteristic for the activation of the at least one auxiliary induction coil for at least one load case is stored. By this means, the at least one auxiliary induction coil can be operated with regard to the load case which is currently present, for example start-up of the processor or standby operation. Also stored in such a look-up table are the appropriate drive data for driving the at least one auxiliary induction coil. If a specific look-up table is compiled for a specific induction heating device, a further advantage results from the fact that the production tolerances for this specific induction heating device, in particular the production tolerances for the primary and auxiliary induction coils and for the thickness and coating of the cylindrical body, can be reduced, since deviations can be compensated for electronically by driving the induction coils appropriately.
Preferably, at least one temperature sensor can also be arranged in the area of the cylindrical body, the control device being designed in such a way that it permits the output signal from the at least one temperature sensor to be taken into account when activating the at least one auxiliary induction coil. If temperature sensors with very short response times, for example non-contacting IR sensors, are used, a very uniform temperature distribution may be achieved on the surface of the cylindrical body. Use is preferably made of three temperature sensors, which are arranged in the two edge regions and in the central region of the cylindrical body.
The present invention likewise comprises a processor for processing a recording medium by using an induction heating device according to the invention. A processor of this type preferably also comprises transport means, in order to transport the recording medium through the processor, exposure means, which are arranged relative to a predetermined section of the cylindrical body in order to expose the recording medium as its rests on this section, and also pressing means, in order to press the recording medium against the outer surface of the cylindrical body for its development.